Fuel cells are highly efficient and clean power generators that directly convert oxygen from air and hydrogen from hydrocarbon fuels such as methanol, natural gas, etc., into electrical energy through electrochemical reactions. Fuel cells were developed as a means of providing power for spacecraft and military purposes in the United States in the 1970s. Since then, numerous research projects have been conducted to develop fuel cells for civilian purposes. Now, in many developed countries, such as the United States, Japan, the European countries, etc., intense research and development is being conducted for practical fuel cells, i.e., generally useful power sources for both military and civilian purposes, for use in everyday life.
A fuel cell can be classified as a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type of fuel cell depending upon the kind of electrolyte used. Among these, polymer electrolyte membrane fuel cells (PEMFCs), have been developed with reduced problems of corrosion or evaporation, superior power characteristics over conventional fuel cells due to their high current density per unit area, and lower operating temperatures. Such fuel cells have advantages in that they can be applied to a wide array of fields, such as for transportable electrical sources for an automobiles, for distributed power such as for houses and public buildings, and for small electrical sources for electronic devices.
A single cell is composed of a solid polymer electrolyte made of a proton exchange membrane, a fuel electrode attached on a side of the solid electrolyte, and an air electrode attached on the other side of the solid electrolyte. Typical fuel cell designs link together these single cells to form a stack to provide a more useful voltage. As a result, a fuel cell stack made by stacking these single cells in many layers is a core component of the fuel cell power plant system that is capable of generating several W to hundreds of kW of electricity.
In a polymer fuel cell power plant system, the performance (or capacity) of the membrane-electrode assembly has a great influence on the electricity generating characteristics. The membrane-electrode assembly described above consists of a polymer electrolyte membrane and a catalyst electrode layer. As the polymer electrolyte membrane, fluorine-based electrolyte membranes such as Nafion™ (DuPont), Flemion (Asahi Glass), Asiplex (Asahi Chemical), and Dow XUS (Dow Chemical) are widely used.
However, commercializing polymer electrolyte membrane fuel-cells as practical power sources has been difficult because the polymer electrolyte membranes currently used are relatively expensive.
In addition, in order to attach the above polymer electrolyte membrane to the catalyst electrode layer, a fluorine-based polymer solution with proton conductivity has usually been used as a binder for manufacturing the catalyst electrode layer. However, such fluorine-based polymer solutions tend to have low proton conductivity when the temperature is above 80° C. and the humidity is below 60% . Furthermore, fluorine-based polymer solutions have a low methanol cross-over. For these reasons, it has been impossible to use fluorine-based polymer solutions in fuel cells operating at temperatures above 100° C .